Process for throttling a compressed gas for evaporative cooling

ABSTRACT

A system and method of cooling a compressed working fluid is disclosed. The method includes compressing the working fluid above its critical pressure point in a compression stage to generate a compressed working fluid at or about local ambient temperature. The compressed working fluid can be cooled to below ambient by throttling a portion of the compressed working fluid to its saturated liquid-vapor state to generate a recycle working fluid. The recycle working fluid may then be atomized using an atomizing nozzle whereby the recycle working fluid evaporates and cools working fluid entering a target compression stage.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 61/376,133 entitled “Process for Throttling aCompressed Gas for Evaporative Cooling,” which was filed on Aug. 23,2010. The contents of the priority application are hereby incorporatedby reference into the present disclosure in their entirety.

BACKGROUND

As a compressor increases the pressure of a working fluid, thetemperature of the working fluid and the components of the compressoralso increases. Increased temperatures oftentimes decrease compressorefficiency by amplifying the amount of work required to compress thefluid. This can result in damage to or premature failure of compressorcomponents. To avoid compressor damage and simultaneously increase itsefficiency, several cooling strategies are typically employed to achievea more iso-thermal compression process.

For example, one common cooling strategy includes a continuous coolingsystem, where there is no direct contact between the incoming workingfluid and the cooling medium. In a typical continuous cooling systemarrangement, a finite number of compression stages are equipped with aseries of external heat exchangers interposed between each stage andconfigured to intercool the working fluid to at or near local ambientconditions. Decreasing the temperature of the cooling medium to belowlocal ambient, however, requires additional work and results in anincreased power demand and decreased efficiency.

Another common compressor cooling strategy is evaporative cooling wherecooling is achieved by directly mixing the working fluid with thecooling medium. In a typical evaporative cooling arrangement, thecooling medium is injected directly into the gas loop of the compressorwhere it is atomized and evaporated into the working fluid. Inoperation, the evaporative cooling strategy relies on the evaporationand adiabatic saturation of the cooling medium to decrease thetemperature of the working fluid.

While there are several compressor cooling strategies known, it isnonetheless desirable to find improved and more efficient methods ofcooling.

SUMMARY

Embodiments of the disclosure may provide a system for cooling acompressed working fluid. The system may include a compressor having aseries of compression stages for compressing a working fluid, the seriesof compression stages including an evaporative compression stage thatcompresses the working fluid to at least a critical pressure, and aseries of heat exchangers fluidly coupled to the series of compressionstages, wherein at least one heat exchanger is interposed between eachcompression stage and configured to decrease a temperature of theworking fluid discharged from a preceding compression stage. The systemmay also include a valve communicably coupled to the at least one heatexchanger following the evaporative compression stage and configured toreceive and throttle a portion of the working fluid as a recycle workingfluid, wherein the valve throttles the recycle working fluid to at leastits saturated liquid-vapor state, and a fogging device fluidly coupledto the valve and a target compression stage, the fogging device beingconfigured to receive the recycle working fluid and evaporatively coolthe working fluid entering the target compression stage.

Embodiments of the disclosure may further provide a method of cooling acompressed working fluid. The method may include compressing a workingfluid in a series of compression stages, the series of compressionstages including an evaporative compression stage that compresses theworking fluid to at least a critical pressure, and cooling the workingfluid in a series of heat exchangers fluidly coupled to the series ofcompression stages, wherein at least one heat exchanger is interposedbetween each compression stage and each heat exchanger is configured todecrease a temperature of the working fluid discharged from a precedingcompression stage. The method may also include throttling a portion ofthe working fluid discharged from the evaporative compression stage toat least its saturated liquid-vapor state with a valve fluidly coupledto the evaporative compression stage, and atomizing the portion of theworking fluid with a fogging device fluidly coupled to the valve and atarget compression stage. The method may further include evaporativelycooling the working fluid entering the target compression stage.

Embodiments of the disclosure may further provide another method ofcooling a working fluid in a compressor. The method may includecompressing the working fluid above a critical pressure in a firstcompression stage to generate a compressed working fluid, and throttlinga portion of the compressed working fluid to its saturated liquid-vaporstate to generate a recycle working fluid. The method may furtherinclude atomizing the recycle working fluid with an atomizing nozzlewhereby the recycle working fluid evaporates and cools, injecting therecycle working fluid at a suction inlet of a target compression stageto cool the working fluid therein.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying Figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale. In fact, the dimensions of the variousfeatures may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates an exemplary system for cooling a compressed workingfluid, according to one or more embodiments disclosed.

FIG. 2 illustrates a representative pressure versus enthalpy diagram forthe working fluid used in the system of FIG. 1, according to one or moreembodiments disclosed.

FIG. 3 illustrates another exemplary system for cooling a compressedgas, according to one or more embodiments disclosed.

FIG. 4 illustrates another representative pressure versus enthalpydiagram for the working fluid used in the system of FIG. 3, according toone or more embodiments disclosed.

FIG. 5 illustrates a flow chart of a method for cooling a working fluid,according to one or more embodiments disclosed.

DETAILED DESCRIPTION

It is to be understood that the following disclosure describes severalexemplary embodiments for implementing different features, structures,or functions of the invention. Exemplary embodiments of components,arrangements, and configurations are described below to simplify thepresent disclosure; however, these exemplary embodiments are providedmerely as examples and are not intended to limit the scope of theinvention. Additionally, the present disclosure may repeat referencenumerals and/or letters in the various exemplary embodiments and acrossthe Figures provided herein. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various exemplary embodiments and/or configurationsdiscussed in the various Figures. Moreover, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed interposing the first and second features, suchthat the first and second features may not be in direct contact.Finally, the exemplary embodiments presented below may be combined inany combination of ways, i.e., any element from one exemplary embodimentmay be used in any other exemplary embodiment, without departing fromthe scope of the disclosure.

Additionally, certain terms are used throughout the followingdescription and claims to refer to particular components. As one skilledin the art will appreciate, various entities may refer to the samecomponent by different names, and as such, the naming convention for theelements described herein is not intended to limit the scope of theinvention, unless otherwise specifically defined herein. Further, thenaming convention used herein is not intended to distinguish betweencomponents that differ in name but not function. Additionally, in thefollowing discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to.” All numericalvalues in this disclosure may be exact or approximate values unlessotherwise specifically stated. Accordingly, various embodiments of thedisclosure may deviate from the numbers, values, and ranges disclosedherein without departing from the intended scope. Furthermore, as it isused in the claims or specification, the term “or” is intended toencompass both exclusive and inclusive cases, i.e., “A or B” is intendedto be synonymous with “at least one of A and B,” unless otherwiseexpressly specified herein.

FIG. 1 illustrates an exemplary system 100 for cooling a compressedworking fluid, according to one or more disclosed embodiments. Thesystem 100 includes a compressor 102, such as a multi-stage compressor,that has a series of compression stages 104 a, 104 b, 104 c, and 104 d.In one or more embodiments, the compressor 102 is a centrifugalcompressor and may form an integral part of an industrial compressionsystem. In other embodiments, the compressor 102 may be an axial orreciprocating compressor. While only four compression stages 104 a-d areillustrated, it will be appreciated that more or less compression stagesmay be implemented without departing from the scope of the disclosure.For instance, embodiments contemplated herein include as many as eightor ten compression stages.

The system 100 may employ a continuous cooling strategy including aseries of heat exchangers 106 a, 106 b, 106 c, and 106 d fluidly coupledto and interposing succeeding compression stages 104 a, 104 b, 104 c,and 104 d, respectively. The heat exchangers 106 a-d may be externalheat exchangers, or they may form an integral part of the compressor 102assembly. The heat exchangers 106 a-d may any type of heat exchangingdevice, such as, but not limited to, direct contact heat exchangers,trim coolers, water-cooled heat exchangers, air-cooled heat exchangers,mechanical refrigeration units, combinations thereof, and the like.While only four heat exchangers 106 a-d are illustrated, it will beappreciated that more or less than four may be implemented withoutdeparting from the scope of the disclosure. For instance, otherembodiments contemplated herein include multiple heat exchangersinterposed between each compression stage 104 a-d.

The system 100 employs an evaporative cooling strategy including atleast one valve 108 and at least one fogging device 110. The valve 108may be a throttling valve such as an expansion valve. In otherembodiments, the valve 108 may be an expander or a turbine configured torecover a portion of power through the expansion of the compressedworking fluid. The fogging device 110 may be configured to convert theworking fluid into a “fog,” or atomized droplets, for injection into atarget compression stage, such as the succeeding fourth compressor stage104 d. To atomize the fluid and generate the fog, a high-pressureatomizing nozzle 112 may be employed within the fogging device 110. Inat least one embodiment, the atomizing nozzle 112 may be operable atpressures ranging from about 7,000 KPa to about 20,000 KPa. It will beappreciated, however, that the operable pressure ranges for theatomizing nozzle 112 may vary as a function of the gas properties. Also,while only one evaporative cooling strategy is depicted in FIG. 1,embodiments contemplated herein include having two or more evaporativecooling strategies implemented in the same compressor 102, withoutdeparting from the scope of the disclosure.

In exemplary operation, a working fluid to be compressed and/or conveyedis introduced into the system 100 and the compressor 102 via line 114.The working fluid may be a compressible gas such as, but not limited to,carbon dioxide (CO₂). The first compression stage 104 a compresses theincoming working fluid, thereby increasing both the pressure and thetemperature of the working fluid. The suction pressure of the succeedingcompression stage (i.e., the second compression stage 104 b) is directlyaffected by its suction temperature. A cooler suction temperature willdemand less power to operate the compression stage for the same massflow to reach the same discharge pressure. Accordingly, in order tomaintain a substantially iso-thermal compression process within thecompressor 102, and thereby maintain or otherwise improve compressorefficiency, the first compression stage 104 a conveys the working fluidto the first heat exchanger 106 a where the temperature of the workingfluid is decreased to at or about ambient temperature.

Once cooled in the first heat exchanger 106 a, the working fluid maythen be discharged to the second compression stage 104 b and subsequentsecond heat exchanger 106 b, where the compression and cooling processis generally repeated. As depicted, similar compression/coolingprocesses are repeated a third and a fourth time in the thirdcompression stage 104 c and third heat exchanger 106 c, and the fourthcompression stage 104 d and fourth heat exchanger 106 d, respectively.The compressor 102 then discharges a compressed working fluid via line107 to be used, for example, in downstream applications. In oneembodiment, downstream applications may include carbon dioxidesequestration and/or storage.

Lowering the working fluid temperature, especially below ambienttemperature, prior to any of the compression stages 104 a-d reduces thetotal amount of compression work required and thereby increases theefficiency of the compressor 102. For example, in order to lower thetemperature of the working fluid to below ambient temperature for thefourth compression stage 104 d, a portion of the working fluiddischarged from the fourth heat exchanger 106 d may be extracted as arecycled working fluid and directed into line 116. The extracted workingfluid may be throttled through the valve 108 to bring the recycleworking fluid to its saturated liquid state, resulting in a liquid orpartially-liquid recycle working fluid discharged into line 118.Throttling the recycle working fluid through the valve 108 may beconfigured to reduce the static pressure of the recycle working fluid toat or near the suction inlet pressure of the fourth compression stage104 d.

The recycle working fluid in line 118 may then be introduced to thefogging device 110 which is fluidly coupled to a target compressionstage, i.e., the compression stage that is to be evaporatively cooled.In the illustrated embodiment, the fogging device 110 is arranged toevaporatively cool the working fluid discharged from the third heatexchanger 106 c before the working fluid is introduced into the inlet ofthe fourth compression stage 104 d. Specifically, the atomizing nozzle112 disposed within the fogging device 110 receives and atomizes therecycle working fluid derived from line 118. Atomizing the recycleworking fluid facilitates its quick evaporation in the presence of theincoming working fluid from the third heat exchanger 106 c, and therebycools the incoming working fluid.

In exemplary operation, the atomizing nozzle 112 may generate a dropletsize and general distribution that promotes the evaporation of thedroplets before the droplets reach downstream impeller blades or othersensitive parts of the succeeding compression stage (e.g., the fourthcompression stage 104 d). As can be appreciated, therefore, atomizationof the fluid substantially prevents machinery erosion and/orrotordynamic vibrational issues.

Referring to FIG. 2, depicted is a representative pressure versusenthalpy diagram 200 for the system 100 as generally described above.The diagram 200 depicts a thermodynamic phase dome 202 corresponding toa working fluid that can be used in the system 100. For instance, thephase dome 202 may be representative of CO₂. The phase dome 202 isgenerally centered around the critical pressure point 204 of the workingfluid, where the saturated liquid and saturated vapor lines meet. Abovethe critical point 204 and outside of the phase dome 202, the workingfluid exists as a dense gas, while below the critical point 204 andinside or below the phase dome 202, the working fluid exists as asaturated liquid-vapor.

As depicted in the diagram 200, the working fluid is compressed andcooled in several stages corresponding to the system 100 describedabove. For example, line 206 a represents compression of the workingfluid in the first compression stage 104 a, and line 208 a representscooling the working fluid in the first heat exchanger 106 a. Likewise,lines 206 b, 206 c, and 206 d represent compression of the working fluidin the second, third, and fourth compression stages 104 b, 104 c, and104 d, respectively, and lines 208 b, 208 c, and 208 d represent coolingthe working fluid in heat exchangers 106 b, 106 c, and 106 d,respectively.

Embodiments of the system 100 described herein take advantage of theworking fluid being compressed above its critical pressure, or otherwiseoutside the thermodynamic phase dome 202. Although an evaporativecooling strategy may be implemented at any point during the compressionprocess, in order to maximize the regenerative cooling effect, theworking fluid throttled through the valve 108 should be at or above thecritical pressure point 204 of the working fluid. Since the fourthcompression stage 104 d (corresponding to line 206 d) compresses theworking fluid to either meet or exceed the critical pressure point 204,it may be characterized as an “evaporative compression stage,” andevaporative cooling of the working fluid may be effectively undertakenthereafter as generally described above.

Consequently, the dashed line 210 in the diagram 200 represents the flowof the recycle working fluid via line 116 as it is throttled through thevalve 108 (FIG. 1) to a saturated liquid state or saturated liquid-vaporstate at or inside the thermodynamic vapor dome 202. It will beappreciated by those skilled in the art that in embodiments employing aturbine in place of the valve 108, as described above, that the dashedline 210 would proceed along an adiabatic curve. In one or moreembodiments, the valve 108 may be configured to throttle the recycleworking fluid to a pressure at or near the suction inlet pressure 212 ofa target compression stage, such as the fourth or evaporativecompression stage 104 d (i.e., line 206 d).

It will be appreciated that the target compression stage may also be theevaporative compression stage itself, as depicted, or any compressionstage that precedes the evaporative compression stage. When theconditions of the recycle working fluid are right (e.g., at or above itscritical pressure point 204), the recycle working fluid may be throttledand then subsequently atomized and injected back into the targetcompression stage using the atomizing nozzle 112 disposed within thefogging device 110 (FIG. 1). As the recycle working fluid is ejectedfrom the atomizing nozzle 112, it quickly evaporates and providesregenerative cooling to the target compression stage which lowers thetemperature of the working fluid below ambient temperature.

Evaporatively cooling the working fluid lowers the inlet temperature forany succeeding compression stages, and thereby reduces the total amountof work required during the compression process. Consequently, eventaking into account the additional work required to re-compress therecycled working fluid and the increase in mass flowrate for succeedingcompression stages, the overall power requirement demand for the system100 is notably lowered, therefore increasing its overall efficiency.

Referring now to FIG. 3, depicted is another exemplary system 300 forcooling a compressed gas, according to embodiments described herein. Insome respects, the system 300 may be substantially similar to the system100 of FIG. 1. As such, FIG. 3 may be best understood with reference toFIG. 1 where like numerals represent like components that will not bedescribed again in detail. Similar to FIG. 1, the system 300 of FIG. 3includes a multi-stage compressor 102 having a series of compressionstages 104 a-d alternatingly interposed by a corresponding series ofheat exchangers 106 a-d. As illustrated, however, the evaporativecooling strategy, including the valve 108 and the fogging device 110,may target the third compression stage 104 c, thereby characterizing thethird compression stage 104 c as the evaporative compression stage. Aswill be described in more detail below, any compression stage thatsuccessfully compresses the working fluid to at or above the criticalpressure of the working fluid may be characterized as the evaporativecompression stage.

The working fluid is introduced into the system 300 via line 114 anddirected to the first compression stage 104 a to compress the workingfluid. In order to maintain at least a substantially iso-thermalcompression process, the first compression stage 104 a may be configuredto convey the working fluid to the first heat exchanger 106 a where thetemperature of the working fluid decreased to at or around ambienttemperature. Once cooled, the working fluid may then be discharged fromthe first heat exchanger 106 a, and the process may be repeated insucceeding compression stages 104 b, 104 c and heat exchangers 106 b,106 c.

A portion of the working fluid discharged from the third heat exchanger106 c may be separated into line 302 as the recycle working fluid. Therecycle working fluid is then throttled to its saturated liquid-vaporstate using the valve 108 and directed to the fogging device 110 vialine 304 as a liquid or partially-liquid recycle working fluid. Afterbeing atomized by the atomizing nozzle 112 disposed within the foggingdevice 110, the recycle working fluid evaporates in the presence of theincoming working fluid from the second heat exchanger 106 b, therebyevaporatively cooling the incoming working fluid to a temperature belowambient.

Referring to FIG. 4, depicted is a pressure versus enthalpy diagram 400for the system 300 generally described above. The diagram 400 includesan exemplary thermodynamic phase dome 402 having a critical pressurepoint 404 corresponding to the working fluid used in the system 300. Theworking fluid is compressed and cooled in several stages correspondingto the system 300 described above. For instance, line 406 a representsthe compression of the working fluid in the first compression stage 104a, and line 408 a represents the working fluid being cooled in the firstheat exchanger 106 a. Likewise, lines 406 b-d represent compression ofthe working fluid in the second, third, and fourth compression stages104 b-d, respectively, and lines 408 b-d represent cooling of theworking fluid in heat exchangers 106 b-d, respectively.

As illustrated, the third compression stage 104 c (corresponding to line406 c in the diagram 400) is capable of compressing the working fluid toeither meet or exceed the critical pressure point 404 of the workingfluid. Consequently, the third compression stage 104 c may be used asthe evaporative compression stage in system 300, and evaporative coolingof the working fluid as described herein may be effectively undertakenat any point thereafter. Thus, the dashed line 410 in the diagram 400represents the flow of the recycle working fluid as it is throttledthrough the valve 108 (FIG. 3) to a saturated liquid-vapor state insidethe thermodynamic vapor dome 402. Again, it will be appreciated by thoseskilled in the art that in embodiments employing a turbine in place ofthe valve 108, that the dashed line 410 would proceed along an adiabaticcurve. The recycle working fluid may be throttled to a suction inletpressure 412 of a desired target compression stage, or the compressionstage where the evaporative cooling is to take place. As illustrated inFIGS. 3 and 4, the target compression stage may be the third compressionstage 104 c (i.e., line 406 c). Consequently, the valve 108 may beconfigured to throttle the recycle working fluid to at or near thesuction inlet pressure 412 of the third compression stage 104 c. Again,it will be appreciated that the target compression stage may be theevaporative compression stage itself or any other compression stage thatprecedes the evaporative compression stage.

The recycle working fluid may then be atomized and injected back intothe working fluid at the third compression stage 104 c via line 406 cusing the atomizing nozzle 112 disposed within the fogging device 110(FIG. 3). As the recycle working fluid is ejected from the atomizingnozzle 112, it evaporatively cools the target compression stage, whichallows the temperature of the working fluid to be lowered below ambient.

Referring now to FIG. 5, illustrated is a flow chart of a method 500 forcooling a working fluid. The method 500 may include injecting a workingfluid into a compressor, as at 502. The working fluid may be compressedin a series of compression stages arranged in the compressor, as at 504.In at least one embodiment, one of the series of compression stages maybe an evaporative compression stage that compresses the working fluid toat least its critical pressure. The working fluid may then be cooled ina series of heat exchangers fluidly coupled to the series of compressionstages, as at 506. In one or more embodiments, at least one heatexchanger is interposed between each compression stage. Moreover, eachheat exchanger may be configured to decrease the temperature of theworking fluid discharged from a preceding compression stage to at ornear ambient temperature. A portion of the working fluid may then bethrottled to at least its saturated liquid-vapor state, as at 508. Theportion of the working fluid may be throttled using a valve fluidlycoupled to a heat exchanger following the evaporative compression stage.In other embodiments, the working fluid is throttled using an expanderor turbine. The portion of the working fluid may then be atomized with afogging device fluidly coupled to both the valve and at least onecompression stage, as at 510, such as a target compression stage. Theworking fluid entering the at least one compression stage may then beevaporatively cooled through evaporative cooling of the throttledportion of the working fluid, as at 512.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the present disclosure. Thoseskilled in the art should appreciate that they may readily use thepresent disclosure as a basis for designing or modifying other processesand structures for carrying out the same purposes and/or achieving thesame advantages of the embodiments introduced herein. Those skilled inthe art should also realize that such equivalent constructions do notdepart from the spirit and scope of the present disclosure, and thatthey may make various changes, substitutions and alterations hereinwithout departing from the spirit and scope of the present disclosure.

1. A system for cooling a compressed working fluid, comprising: acompressor having a series of compression stages for compressing aworking fluid, the series of compression stages including an evaporativecompression stage and a target compression stage, wherein theevaporative compression stage compresses the working fluid to at least acritical pressure; a series of heat exchangers fluidly coupled to theseries of compression stages such that at least one heat exchangerinterposes adjacent compression stages, the series of heat exchangersbeing configured to decrease a temperature of the working fluiddischarged from a preceding compression stage; a valve fluidly coupledto a heat exchanger following the evaporative compression stage, thevalve being configured to throttle a portion of the working fluid to atleast its saturated liquid-vapor state; and a fogging device fluidlycoupled to the valve and the target compression stage, the foggingdevice being configured to evaporate the portion of the working fluid toevaporatively cool the working fluid entering the target compressionstage.
 2. The system of claim 1, wherein the compressor is a centrifugalcompressor.
 3. The system of claim 1, wherein the working fluid iscarbon dioxide.
 4. The system of claim 1, wherein the series of heatexchangers are water-cooled or air-cooled heat exchangers.
 5. The systemof claim 1, wherein the series of heat exchangers are configured toreduce the temperature of the working fluid to at or near ambienttemperature.
 6. The system of claim 1, wherein the valve furtherthrottles the portion of the working fluid to at or near a suctionpressure of the target compression stage.
 7. The system of claim 1,wherein the fogging device has an atomizing nozzle that atomizes theportion of the working fluid in the presence of the working fluid. 8.The system of claim 7, wherein the fogging device reduces thetemperature of the working fluid to below ambient temperature.
 9. Thesystem of claim 1, wherein the evaporative compression stage and thetarget compression stage are the same compression stage.
 10. A method ofcooling a compressed working fluid, comprising: compressing a workingfluid in a series of compression stages, the series of compressionstages including an evaporative compression stage and a targetcompression stage; cooling the working fluid in a series of heatexchangers fluidly coupled to the series of compression stages, whereinat least one heat exchanger is interposed between each compression stageand each heat exchanger is configured to decrease a temperature of theworking fluid discharged from a preceding compression stage; compressingthe working fluid to at least a critical pressure in the evaporativecompression stage; throttling a portion of the working fluid dischargedfrom the evaporative compression stage with a valve fluidly coupled tothe evaporative compression stage, the portion of the working fluidbeing throttled to at least its saturated liquid-vapor state; andatomizing the portion of the working fluid with a fogging device fluidlycoupled to the valve and the target compression stage, whereby theportion of the working fluid evaporates and cools the working fluidentering the target compression stage.
 11. The method of claim 10,wherein cooling the working fluid in a series of heat exchangers furthercomprises cooling the working fluid discharged from a precedingcompression stage to at or near ambient temperature.
 12. The method ofclaim 10, further comprising throttling the portion of the working fluidto at or near a suction pressure of the target compression stage. 13.The method of claim 10, wherein the fogging device comprises anatomizing nozzle.
 14. The method of claim 13, further comprisingatomizing the portion of the working fluid with the atomizing nozzle.15. The method of claim 10, further comprising evaporatively cooling theworking fluid entering the target compression stage to below ambienttemperature.
 16. A method of cooling a working fluid in a compressor,comprising: compressing the working fluid above a critical pressure in afirst compression stage to generate a compressed working fluid;throttling a portion of the compressed working fluid to its saturatedliquid-vapor state to generate a recycle working fluid; atomizing therecycle working fluid with an atomizing nozzle whereby the recycleworking fluid evaporates and cools; and injecting the recycle workingfluid at a suction inlet of a target compression stage to cool theworking fluid therein.
 17. The method of claim 16, wherein the workingfluid is carbon dioxide.
 18. The method of claim 16, further comprisingthrottling the portion of the compressed working fluid to at or near asuction pressure of the second compression stage.
 19. The method ofclaim 16, wherein the target compression stage is the first compressionstage.
 20. The method of claim 16, further comprising cooling theworking fluid entering the second compression stage to below ambienttemperature. 21-37. (canceled)